feat(ui): D.2b inventory finish — UiItemList mouse-wheel scroll

OnEvent handles the Scroll wheel in grid mode (mirrors UiText's sign), driving
the shared UiScrollable. The bound gutter scrollbar (next task) is the primary
scroll affordance; the wheel is the convenience path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/UiItemListScrollTests.cs

@@ -51,4 +51,15 @@ public sealed class UiItemListScrollTests
         Assert.Equal(0f, list.GetItem(6)!.Top);         // row 1 now at the view top
         Assert.True(list.GetItem(18)!.Visible);         // row 3 now visible
     }
+
+    [Fact]
+    public void Wheel_down_scrolls_toward_the_bottom()
+    {
+        var list = Grid(30);                            // max scroll 64, LineHeight 32
+        // Silk wheel +Y = up/older; UiText negates Data0. Wheel DOWN (Data0 < 0) → +ScrollY.
+        var e = new UiEvent(0u, null, UiEventType.Scroll, Data0: -1);
+        bool handled = list.OnEvent(e);
+        Assert.True(handled);
+        Assert.Equal(32, list.Scroll.ScrollY);          // one line (32px) down
+    }
 }